Transformers include a primary coil to which power is input, a secondary coil from which power is output and a core (iron core) around which the primary and secondary coils are coiled to be magnetically coupled. The relative positions of the primary and secondary coils and the core of, for instance, a boosting transformer are of great significance and have to be accurately maintained in order to stably obtain a predetermined inductance performance (magnetic coupling degree) required for a boost operation. Further, the primary and secondary coils and the core have to be mutually insulated and have to be efficiently cooled because the primary and secondary coils and the core sometimes generate heat due to electric loss and magnetic loss during the boost operation (i.e. when being energized). When the above requirements are not met, the transformer may sometimes be inoperative. Further, in case of insufficient cooling, an operative range capable of stable boosting may be narrowed.
Accordingly, as disclosed in Patent Literature 1, a typical transformer includes a plate piece made of a composite material of glass and epoxy (usually referred to as glass epoxy) of a predetermined thickness inserted between the primary and secondary coils and the core. A similar plate piece is inserted between the primary and secondary coils. When a transformer is assembled, a sub-assembly in which the mutual positions of the primary and secondary coils and the core are accurately held is initially made and a thermosetting fluid resin (referred to as a mold resin hereinafter) is put into a case housing the sub-assembly, which is thermally cured to completely hold the relative positions of the primary and secondary coils and the core. The mold resin is also required for preventing loss of insulating function due to invasion of foreign substances or moisture into the case.
It should be noted that the case in which the sub-assembly is housed is connected to a heat sink via a screw and the like, where the heat generated by the sub-assembly is transmitted to the heat sink to be radiated. Accordingly, the case preferably has good thermal conductivity and thus is made of aluminum casting. Further, since the core is the component of the sub-assembly that is adapted to be brought into contact with the case with the largest area in view of the magnetic properties, the bottom side of the core is made to be a flat surface and a core-contacting surface facing the flat surface is also made flat. Since the flat surfaces are closely contacted, the heat of the sub-assembly is efficiently transferred from the core to the case and is radiated via the heat sink without impairing the magnetic properties.